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(#) View Holder Candidates

!!! WARNING: View Holder Candidates
   This is a warning.

Id
:   `ViewHolder`
Summary
:   View Holder Candidates
Severity
:   Warning
Category
:   Performance
Platform
:   Android
Vendor
:   Android Open Source Project
Feedback
:   https://issuetracker.google.com/issues/new?component=192708
Since
:   Initial
Affects
:   Kotlin and Java files
Editing
:   This check runs on the fly in the IDE editor
See
:   https://developer.android.com/guide/topics/ui/layout/recyclerview#ViewHolder
Implementation
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-checks/src/main/java/com/android/tools/lint/checks/ViewHolderDetector.java)
Tests
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/ViewHolderDetectorTest.kt)
Copyright Year
:   2014

When implementing a view Adapter, you should avoid unconditionally
inflating a new layout; if an available item is passed in for reuse, you
should try to use that one instead. This helps make for example
`ListView` scrolling much smoother.

(##) Example

Here is an example of lint warnings produced by this check:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text
src/test/pkg/ViewHolderTest.java:42:Warning: Unconditional layout
inflation from view adapter: Should use View Holder pattern (use
recycled view passed into this method as the second parameter) for
smoother scrolling [ViewHolder]
    convertView = mInflater.inflate(R.layout.your_layout, null);
                  ---------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here is the source file referenced above:

`src/test/pkg/ViewHolderTest.java`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers
package test.pkg;

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.LinearLayout;
import android.widget.TextView;

import java.util.ArrayList;

@SuppressWarnings({"ConstantConditions", "UnusedDeclaration"})
public abstract class ViewHolderTest extends BaseAdapter {
    @Override
    public int getCount() {
        return 0;
    }

    @Override
    public Object getItem(int position) {
        return null;
    }

    @Override
    public long getItemId(int position) {
        return 0;
    }

    public static class Adapter1 extends ViewHolderTest {
        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            return null;
        }
    }

    public static class Adapter2 extends ViewHolderTest {
        LayoutInflater mInflater;

        public View getView(int position, View convertView, ViewGroup parent) {
            // Should use View Holder pattern here
            convertView = mInflater.inflate(R.layout.your_layout, null);

            TextView text = (TextView) convertView.findViewById(R.id.text);
            text.setText("Position " + position);

            return convertView;
        }
    }

    public static class Adapter3 extends ViewHolderTest {
        LayoutInflater mInflater;

        public View getView(int position, View convertView, ViewGroup parent) {
            // Already using View Holder pattern
            if (convertView == null) {
                convertView = mInflater.inflate(R.layout.your_layout, null);
            }

            TextView text = (TextView) convertView.findViewById(R.id.text);
            text.setText("Position " + position);

            return convertView;
        }
    }

    public static class Adapter4 extends ViewHolderTest {
        LayoutInflater mInflater;

        public View getView(int position, View convertView, ViewGroup parent) {
            // Already using View Holder pattern
            //noinspection StatementWithEmptyBody
            if (convertView != null) {
            } else {
                convertView = mInflater.inflate(R.layout.your_layout, null);
            }

            TextView text = (TextView) convertView.findViewById(R.id.text);
            text.setText("Position " + position);

            return convertView;
        }
    }

    public static class Adapter5 extends ViewHolderTest {
        LayoutInflater mInflater;

        public View getView(int position, View convertView, ViewGroup parent) {
            // Already using View Holder pattern
            convertView = convertView == null ? mInflater.inflate(R.layout.your_layout, null) : convertView;

            TextView text = (TextView) convertView.findViewById(R.id.text);
            text.setText("Position " + position);

            return convertView;
        }
    }

    public static class Adapter6 extends ViewHolderTest {
        private Context mContext;
        private LayoutInflater mLayoutInflator;
        private ArrayList<Double> mLapTimes;

        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            if (mLayoutInflator == null)
                mLayoutInflator = (LayoutInflater) mContext.getSystemService(Context.LAYOUT_INFLATER_SERVICE);

            View v = convertView;
            if (v == null) v = mLayoutInflator.inflate(R.layout.your_layout, null);

            LinearLayout listItemHolder = (LinearLayout) v.findViewById(R.id.laptimes_list_item_holder);
            listItemHolder.removeAllViews();

            for (int i = 0; i < mLapTimes.size(); i++) {
                View lapItemView = mLayoutInflator.inflate(R.layout.laptime_item, null);
                if (i == 0) {
                    TextView t = (TextView) lapItemView.findViewById(R.id.laptime_text);
                    //t.setText(TimeUtils.createStyledSpannableString(mContext, mLapTimes.get(i), true));
                }

                TextView t2 = (TextView) lapItemView.findViewById(R.id.laptime_text2);
                if (i < mLapTimes.size() - 1 && mLapTimes.size() > 1) {
                    double laptime = mLapTimes.get(i) - mLapTimes.get(i + 1);
                    if (laptime < 0) laptime = mLapTimes.get(i);
                    //t2.setText(TimeUtils.createStyledSpannableString(mContext, laptime, true));
                } else {
                    //t2.setText(TimeUtils.createStyledSpannableString(mContext, mLapTimes.get(i), true));
                }

                listItemHolder.addView(lapItemView);

            }
            return v;
        }
    }

    public static class Adapter7 extends ViewHolderTest {
        LayoutInflater inflater;

        @Override
        public View getView(final int position, final View convertView, final ViewGroup parent) {
            View rootView = convertView;
            final int itemViewType = getItemViewType(position);
            switch (itemViewType) {
                case 0:
                    if (rootView != null)
                        return rootView;
                    rootView = inflater.inflate(android.R.layout.simple_list_item_1, parent, false);
                    break;
            }
            return rootView;
        }
    }

    public static final class R {
        public static final class layout {
            public static final int your_layout = 0x7f0a0000;
            public static final int laptime_item = 0x7f0a0001;
        }
        public static final class id {
            public static final int text = 0x7f0a0000;
            public static final int laptimes_list_item_holder = 0x7f0a0001;
            public static final int laptime_text = 0x7f0a0002;
            public static final int laptime_text2 = 0x7f0a0003;
        }
    }
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can also visit the
[source code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/ViewHolderDetectorTest.kt)
for the unit tests for this check to see additional scenarios.

The above example was automatically extracted from the first unit test
found for this lint check, `ViewHolderDetector.testBasic`.
To report a problem with this extracted sample, visit
https://issuetracker.google.com/issues/new?component=192708.

(##) Suppressing

You can suppress false positives using one of the following mechanisms:

* Using a suppression annotation like this on the enclosing
  element:

  ```kt
  // Kotlin
  @Suppress("ViewHolder")
  fun method() {
     problematicStatement()
  }
  ```

  or

  ```java
  // Java
  @SuppressWarnings("ViewHolder")
  void method() {
     problematicStatement();
  }
  ```

* Using a suppression comment like this on the line above:

  ```kt
  //noinspection ViewHolder
  problematicStatement()
  ```

* Using a special `lint.xml` file in the source tree which turns off
  the check in that folder and any sub folder. A simple file might look
  like this:
  ```xml
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;lint&gt;
      &lt;issue id="ViewHolder" severity="ignore" /&gt;
  &lt;/lint&gt;
  ```
  Instead of `ignore` you can also change the severity here, for
  example from `error` to `warning`. You can find additional
  documentation on how to filter issues by path, regular expression and
  so on
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html).

* In Gradle projects, using the DSL syntax to configure lint. For
  example, you can use something like
  ```gradle
  lintOptions {
      disable 'ViewHolder'
  }
  ```
  In Android projects this should be nested inside an `android { }`
  block.

* For manual invocations of `lint`, using the `--ignore` flag:
  ```
  $ lint --ignore ViewHolder ...`
  ```

* Last, but not least, using baselines, as discussed
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).

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